Triple integrals. Variation

Part (c) in Figure 2.85 illustrates a triple cascade loop, where a temperature controller is the slave of an analyzer controller while the reflux flow is cascaded to temperature. Because temperature is an indicator of composition at constant pressure, the analyzer controller serves only to correct for variations in feed composition. Cascade loops will work only if the slave is faster than the master, which adjusts its set point. Another important consideration in all cascade systems (not shown in Figure 2.85) is that an external reset is needed to prevent the integral mode in the master from saturating, when that output is blocked from reaching and modulating the set point of the slave (when the slave is switched to local set point). 

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